Image intensifier tubes are well known in the industry by their commonly used names, based on the generic generation from which their design came into being. The tubes have evolved from Generation 0 to the current Generation III. These tubes have typically been produced in both 18 mm and 25 mm diameter formats.
A significant portion of the military and commercial night vision equipment currently in use was designed to physically accommodate a 25 mm format Generation II (Gen II) image intensifier tube. The military equipment that uses the Gen II tube, includes Individual Served Weapon Sights, Crew Served Weapon Sights, night vision devices that facilitate the operation of motorcraft in low light conditions and for other applications as well.
The Gen II image intensifier tube conforms to very detailed U.S. military specifications and is identified by its U.S. military part number: MX-9644. The performance of the Gen II image intensifier tube is no longer the state of the art. The Gen II image intensifier tube is an inverter tube and exhibits a gain at 6.times.10.sup.-6 foot candles input of from 20,000 to 70,000 with a typical gain of 50,000. The photocathode of a Gen II tube exhibits a luminous sensitivity of approximately 325 microamps per lumen at 2856.degree. K. The Gen II image intensifier tube exhibits a signal-to-noise ratio of approximately 4:1 and a resolution of twenty eight line pairs per millimeter (lp/mm).
A higher performance image intensifier tube has been developed in the Generation III (Gen III) proximity focussed image intensifier tube. A Gen III image intensifier tube employs a gallium arsenide photocathode that has an improved photosensitivity that operates at starlight levels and below. A Gen III image intensifier device, with a glass output screen, exhibits a luminous gain in the range of 20,000 to 70,000 at 2.0.times.10.sup.-6 foot candles. The sensitivity of the Gen III photocathode is over 1000 microamps per lumen at 2856.degree. K., which is more than three times that of the Gen II tube. The signal-to-noise ratio has been increased to approximately 16:1. In addition, the resolution has been increased to 36-40 lp/mm.
Generation III (Gen III) is a generic term established by the United States Department of the Army at its Night Vision Labs at Fort Bellmore. The Gen III image intensifier tube is identified by Military Part Number MX-10160. The criteria set forth by the Department of the Army in distinguishing a Gen III tube from a MX-9644 Gen II tube is that a Gen III tube utilizes a gallium arsenide photocathode and an ion barrier film over the microchannel plate, while the Gen II tube does not. The form and function of the Gen III tube currently manufactured by ITT corp, the assignee herein, is described within U.S. patent application Ser. No. 08/020,650, filed Feb. 22, 1993 and entitled IMAGE INTENSIFIER TUBE. Furthermore, the structure and function of a Gen III image intensifier tube is available in the full military specification MX-10130.
In view of the above performance statistics it should be obvious that the Gen III image intensifier tube is more desirable than the Gen II image intensifier tube, and the Gen III tube should be substituted for the Gen II tube wherever possible. However, certain problems arise when such a substitution is attempted. As has been previously mentioned, many night vision applications were designed and built around the Gen II image intensifier tube. In such applications the size and shape of the night vision device was formed to enclose the MX-9644 Gen II tube. Additionally, the objective lens optics and the eyepiece optics were designed to complement the input and output of the Gen II tube.
In FIG. 1 there is shown a typical night vision application of the Gen II image intensifier tube 10. Positioned at the pupil of the Gen II image intensifier tube 10 is an objective lens assembly 12. Positioned at the rear of the Gen II image intensifier tube 10 is an eyepiece lens assembly 14. The shown optical assembly can represent any night vision application that utilizes the MX-9644 Gen II image intensifier tube. In operation, the objective lens assembly 12 is directed at a target object A1. The objective lens assembly 12 presents an inverted image A2 to the pupil of the Gen II image intensifier tube 10. In turn, the Gen II image intensifier tube inverts the image A2 to form an upright visible image A3 to the pupil of the eyepiece lens assembly 14. Finally, the eyepiece lens assembly 14 presents an upright image A4 to the eye of the observer.
When retrofitting a Generation III image intensifier tube into a Generation II application, two problems occur. First is the size differential. A Gen II image intensifier tube has a length L of approximately 77 mm and has a nominal diameter D of approximately 62 mm. A Gen III image intensifier tube has a length of 16.4 mm and also embodies a smaller nominal diameter. Such a size differential obviously affects the focal positions of the objective lens assembly 12 and the eyepiece lens assembly 14, since both can no longer be properly focussed in relation to the undersized Gen III tube.
The second problem that occurs when retrofitting a Gen III tube into a Gen II tube application, is that the MX-9644 Gen II image intensifier tube inverts the image, whereas the 25 mm format Gen III image intensifier tubes typically do not. Consequently, if a Gen III tube is directly substituted for a Gen II tube, the image viewed by an observer would be inverted.
To solve the above problems of retrofitting a Gen III image intensifier tube into a Gen II tube application, two approaches have been tried. Referring to FIG. 2 the first approach is illustrated. In this prior art embodiment, a Gen III image intensifier tube 16 and a Generation I (Gen I) image intensifier tube 18 are placed in series. The attachment of the Gen I tube 18 to the Gen III tube 16 gives the combined assembly approximately the same length as the MX-9644 Gen II image intensifier tube. As such, the pupil of the Gen III tube is properly distanced from the objective lens assembly 12 and pupil of the eyepiece lens assembly 14 is properly distanced from the output screen of the Gen I tube 18. The problem of inversion is also solved by the combined Gen III tube and Gen I tube assembly, since the Gen I tube 18 reinverts the inverted image B from the objective lens, which is passed through the Gen III tube 16 without inversion. As such, the Gen III and Gen I tube assembly provides the same upright image as would a lone Gen II image intensifier tube.
The combined Gen III and Gen I tube assembly does have disadvantages. The use of adjacent image intensifier tubes produces significant optical losses in the fiber optic plates added to the interface between the two tubes. Similarly, optical losses occur due to the limited MTF of the Gen I electron optics. Since two tubes are used, a more complicated power supply is needed to operate both tubes. Gen I tubes are expensive and becoming increasingly difficult to find because of their obsolescence in favor of the Gen II and Gen III image intensifier tubes. Additionally, by combining a Gen III tube with a Gen I tube, two vacuum envelopes are now included in one application. A consequence of two vacuum envelopes is a shorter mean time between failure (MTBF) and the added expense of repair and maintenance.
The second approach used to retrofit Gen III image intensifier tubes into a specific Gen II tube application is set forth in U.S. Pat. No. 5,029,963 entitled "REPLACEMENT DEVICE FOR A DRIVER'S VIEWER" to Naselli et al and assigned to ITT Corporation, the assignee herein. The approach of the Naselli patent is illustrated in FIG. 3. Referring to FIG. 3 it can be seen that the Naselli patent uses a Gen III image intensifier tube 16 coupled with an inverter lens assembly 20. The inverter lens assembly 20 reinverts the inverted image A2 from the objective lens passing through the Gen III tube producing the inverted image A3 needed at the pupil of the eyepiece lens assembly 14. The inverter lens assembly 20 is arranged so as to compensate for the undersized shape of the Gen III tube and to provide a properly oriented and focussed image to the eyepiece lens assembly 14. In this application, the relay lens magnifies the image format from 25 mm to 46 mm diameter and presents the image to the eyepiece.
As has been previously mentioned, the MX-9644 Gen II image intensifier tube has been the tube of choice in many night vision applications. Those applications include, but are not limited to, Model AN/PVS-4 Individual Served Weapon Sight, the Model AN/TVS-5 Crew Served Weapon Sight, the Model M-36 Night Vision Elbow for tanks, the Model AN/VVS-2 Drivers Viewer and various commercial night vision systems. Although all the above systems are similar in their function, they do not employ identical optics. As such, the unadjustable optics of a single form of the referenced prior art cannot be retrofitted into the variety of existing applications. The reference prior art retrofitting techniques must be custom made for each and every application. For example, the above-noted Naselli patent discloses a retrofit system for the Model AN/VVS-2 Driver Viewer and discloses an optical arrangement specifically dedicated to that system. Additionally, U.S. patent application Ser. No. 07/785,048, to Phillips, filed Oct. 30, 1991 and entitled "TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING" discloses a dedicated optical system that employs a Gen III image intensifier tube in an Individual Served Weapon Sight. These applications are custom made for a specific device and cannot be exchanged into other devices.
It is therefore a primary objective of this invention to provide an image intensifier system that includes a Gen III image intensifier tube that can be retrofitted into a specific MX-9644 Gen II image intensifier tube application.